1. Field of the Invention
The present invention relates generally to a V/I source and more particularly, to automatic test systems for electronic devices containing a V/I source. The invention furthermore relates to a method for forming a V/I source and a method for testing an electronic device. Specifically, the V/I source of the present invention is capable of selectively providing predetermined output loads or output voltages to a device under test (DUT).
2. Description of Related Art
Automatic test equipment is frequently employed to run diagnostic tests on integrated circuit devices. In this respect, the test equipment for example ensures that integrated circuits under test provide the proper output voltages in response to a given input by the test equipment during a functional test. The test equipment can also provide parametric tests for determining electrical characteristics of a device such as resistance and leakage.
Automatic test systems commonly employ circuits for communicating electronic signals with devices to be tested. Devices under test (DUT's) typically include many input/output nodes having similar electronic interface characteristics. Therefore, tester circuits that communicate with these nodes tend to be identical. Testers generally include tens or even hundreds or more of identical, general-purpose interface circuits known in the art as “pin electronics” circuits. Pin electronics circuits, also called “channels,” can differ widely in construction, but most commonly include three basic elements: a driver, a detector, and an active load.
The driver is a source of electronic signals. Typically, drivers are optimized for producing digital signals, which are either high (logic 1) or low (logic 0). Drivers are commonly designed to switch their outputs between predetermined high and low levels under strict timing control.
The detector is a receiver of electronic signals. It receives a signal from the DUT and indicates the state of that signal at particular instants in time.
The active load is a provider/dissipater of electrical energy. The active load subjects output signals of the DUT to loading conditions (e.g., sourcing or sinking current) that are similar to those expected during the DUT's normal operating conditions. Such active loads are generally programmable and the levels of currents they provide/absorb (source and/or sink) can be adjusted.
Test systems generally include timing circuitry for synchronizing drivers, detectors, and active loads. Signals are generated, signals are detected, and loads are applied under strict timing control.
FIG. 1 shows a simplified schematic of a pin electronics channel 110 of the type described above, wherein the channel 110 is connected to a pin of a DUT 120 for performing tests on the DUT. The channel 110 includes a driver 112, a detector 114, and an active load 116. In practice several channels 110 may be connected to a DUT, depending on the amount of driver signals required and the amount of output signals to be detected. Each channel 110 may selectively act as a driver or a detector/load. In the configuration shown in FIG. 1, two channels 110 are provided, wherein for the upper one acts as a driver, while the lower one acts as a detector, as will be described above. The driver 112 of the upper channel 110 supplies a test signal to an input circuit 122 of the DUT, such as the input of a latch. The detector 114 of the lower channel 110 receives an output signal from an output circuit 124 of the DUT, such as the output of a latch. The active load 116 of the lower channel typically also receives the output signal and depending on the output signal absorbs current from the output circuit 124 or supplies current thereto. Test systems generally include a great number of channels like the channel 110, and those channels can be connected to large numbers of input and output circuits of a DUT. This arrangement is merely an example of how a driver, detector, and active load can be used when testing a device. Other arrangements are common.
While channels 110 may be used for functional tests of a DUT, they are typically not capable of performing parametric tests thereon. Thus, in the past, separate parametric measuring units (PMU's) 130 have been used. Inasmuch as functional tests can typically be performed much faster than parametric tests, a single PMU may be assigned to several pins of a DUT 120. The PMU 130 may be connected to a DUT via respective switches 135, 136. During parametric measurements it may be required to disconnect the channels 110 from the DUT, which may be achieved via respective switches 140.
FIG. 2 shows a more detailed view of a typical active load 116 used in automatic test equipment. The active load 116 includes a pair of programmable current sources 210 and 212 that connect to opposite ends of a diode quad 216. One side of the diode quad 216 receives a programmable signal, VCOM, via a respective device 214. The other side of the diode quad (node 220) is adapted to receive a signal to be loaded from a DUT.
The active load 216 operates generally as follows. Assuming that the currents from the current sources 210 and 212 are equal and the node 220 is open, all current flowing into the top of the diode quad 216 flows out the bottom of the diode quad 216. The current divides approximately equally between the left and right sides of the diode quad, i.e., the same current flows through diodes 216a and 216d as flows through diodes 216b and 216c. Because all diodes 216a-216d are conductive, the voltage VCOM applied to the left of the diode quad is approximately duplicated to the right of the diode quad.
If an output signal, such as from a DUT, is now applied to node 220, the balanced state of the diode quad is disturbed. When the voltage at node 220 exceeds VCOM, diode 216a is cut off (reverse-biased) and the current source 212 sinks all of its current from the DUT via diode 216b. Diode 216c is also cut off, and the current source 210 supplies all of its current to VCOM. When the voltage at node 220 is less than VCOM, diodes 216b and 216d are cut off. All current from the current source 210 is sourced to the DUT, and all the current to the current source 212 is sourced from VCOM.
The effect of this behavior is that the DUT sources the current of the current source 212 when its output is higher than VCOM, and sinks the current of current source 210 when its output is lower than VCOM.
The currents of the current sources 210 and 212 need not be the same. For instance, if the current through the current source 210 is greater than the current through the current source 212, the DUT is forced to sink a greater current when outputting a low signal than it is forced to source when outputting a high signal.
Such a load is well suited for loading the output of a DUT for functional tests thereon. Such a load, however, is not well suited for parametric measurements. During parametric measurements it may for example be necessary to force a predetermined current through a DUT (for example a diode) and to measure the voltage drop there across (force current/measure voltage). Additionally it may be necessary to create a predetermined Voltage across a DUT (for example a resistor or a reverse diode) and to measure the current or leaking current flowing therethrough (force voltage/measure current). Though the known load circuit is capable of providing (forcing) a predetermined current, it is not capable of providing (forcing) a predetermined voltage to a DUT. Therefore, different and separate circuits, especially different driver and load circuits had to be provided for different types of tests, leading to a high count of components in the automatic test equipment.
Furthermore, in the force current/measure voltage situation it may be necessary to limit the maximum voltage applied across the DUT, in order to protect the DUT and the automatic test equipment (e.g. in an open circuit situation).
In the force voltage/measure current situation it is necessary to limit the current flowing through the DUT, in order to protect the DUT and the automatic test equipment (e.g. in a short circuit situation).